Traditionally, chlorofluorocarbon (CFC) and hydrochlorofluorocarbon (HCFC) type refrigerants, such as CFC-11 (trichloromonofluoromethane), CFC-12 (dichlorodifluoromethane) and HCFC-22 (monochlorodifluoromethane) among others, have been used as refrigerants in refrigerators, air conditioners, chillers, commercial buildings and other appliances. These chlorine-based refrigerants are believed to destroy the ozone layer and therefore their use is to be gradually eliminated by 1996, under a recent protocol signed in Montreal, Canada by representatives of 167 countries of the world.
Chlorine-free hydrogen-containing halocarbons have already been introduced to replace CFC- and HCFC-type refrigerants. Hydrofluorocarbons (HFC), such as HFC-134 (1,1,2,2-tetrafluoroethane) and HFC-134a (1,1,1,2-tetrafluorethane), are considered to be direct replacements for CFC-12 (also known as R-12) refrigerant. The cooling (thermodynamic) properties of HFC-134a are similar to those of the R-12 product in many applications and HFC-134a appears to have emerged as the currently preferred HFC refrigerant.
Historically, mineral oils, particularly naphthenic mineral oils, and alkylbenzenes, have been used as lubricants with the CFC-type refrigerants. Such mineral oils, however, exhibit poor miscibility with HFC-type refrigerants. The resulting HFC/mineral oil mixture has been found to separate into two layers at ambient temperature. This results in the oil clogging in the cold temperature (evaporators) areas, thus restricting the refrigerant flow and causing poor oil return to the compressor, and it results in reduced efficiency. The lack of an effective lubricant to the compressor can also cause bearing seizure, and eventually compressor breakdown will occur.
Synthetic oils, such as polyalkylene glycol- and polyol ester-type refrigeration oils, have heretofore been introduced as lubricants for HFC-based systems. They have excellent miscibility with HFC-134a. See, for example, U.S. Pat. Nos. 4,948,525 and 4,755,316, which are hereby incorporated herein by reference in their entirety. These synthetic oils perform well in lubricating the compressor bearings.
In addition to the aforementioned problems with using naphthenic-based mineral oils as lubricants with HFC-type refrigerants, they further cannot be used as a compressor break-in lubricant or general purpose lubricating preservative oil for parts during compressor assembly. Although the amount of break-in lubricant left in the compressor after break-in is small, even such small amounts can cause miscibility and or thermal stability problems in systems using HFC-type refrigerants and synthetic polyol ester lubricants.
Using a synthetic polyol ester break-in lubricant or parts lubricant avoids compatibility problems caused by the HFC-type refrigerants; however, polyol esters are hygroscopic. The compressor parts that have been lubricated with oils are exposed to the atmosphere for an extended period of time during compressor assembly. The break-in lubricant is also exposed to the atmosphere during repeated use of the same oil for several break-ins, as is the normal procedure. Hygroscopic oils, such as polyol esters, will adsorb moisture from the atmosphere. Adsorbed moisture is thus introduced into the compressor, which can cause corrosion of compressor parts. Due to their hygroscopic nature, polyol esters are therefore not suitable for use as a break-in lubricant or general purpose parts lubricant.
Thus, there is a need for a lubricant for use with systems using HFC-type refrigerants and synthetic polyol ester lubricants.